1. Field of the Invention
The present invention relates to a spark plug having a combustion pressure detecting function in which an annular pressure sensor for measuring the combustion pressure of an internal combustion engine is tightly fastened together with a gasket when attaching the spark plug.
2. Description of the Related Art
A spark plug equipped with a pressure detecting device (pressure sensor) incorporating a piezoelectric element, proposed in JP-UM-A-61-57830, is among known spark plugs having a combustion pressure detecting function.
The pressure detecting device is fitted so as to contact the seat surface of a brim portion formed behind the rear end of a male thread of the metal shell of the spark plug, and is attached to an internal combustion engine (cylinder head) together with the spark plug. To prevent gas leakage while running the internal combustion engine, the pressure detecting device is attached such that an annular, flat gasket made from copper or a copper-based alloy is disposed between the seat surface of the metal shell and the pressure detecting device. The piezoelectric element in the pressure detecting device is preloaded (i.e., subjected to an applied pressure) when a fastening torque is applied in attaching the spark plug (JP-A-2002-243569).
The pressure detecting device operates in the following manner. When an air-fuel mixed gas explodes in the combustion chamber of an internal combustion engine, the internal pressure of the combustion chamber increases. As a result, the spark plug that is attached to the cylinder head is slightly displaced in an axial direction. The preload pressure on the piezoelectric element is varied by displacement of the spark plug, and the resulting change is detected as a sensor output.
3. Problems to be Solved by the Invention
JP-UM-A-61-57830 discloses that the annular, flat gasket made of the above-mentioned material is an improvement over a previous S-shaped or U-shaped gasket made of iron from which a prescribed output cannot be obtained and which varies to a large extent with respect to the fastening torque. Although JP-A-2002-243569 suggests the use of a gasket made of a copper alloy, it does not disclose the characteristics required of a gasket or means for attaining such characteristics.
In recent years, combustion pressure has increased as engine output power has increased, and a highly accurate sensor output is required for engine control based on combustion pressure. For use in such engines, gaskets made of the above-mentioned conventional materials may not be able to provide as high a sealing performance, anti-loosening performance, etc., as needed. If the gasket is loosened or if gas leakage occurs, the displacement caused by the combustion pressure becomes very small. As a result, the sensor output becomes small and hence the combustion pressure or the combustion state cannot be detected accurately. In addition, gas leakage lowers the output power and fuel efficiency of the engine.
After conducting diligent studies, the present inventors found that a gasket having a large spring constant, a large true contact area, a small gasket displacement and good stress relaxation characteristics (i.e., a small stress relaxation factor) are important for attaining the requisite performance.